In a display of public emotion and solidarity not witnessed since the capital's 1944 liberation from Nazi Germany, 1.5 million French people poured onto the streets of Paris to mourn the 17 killed in three attacks on satirical weekly Charlie Hebdo, a Jewish deli and a policewoman on patrol.
"Proud to be French" was the refrain of countless tweets from those at rallies which brought together at least 3.7 million across the country from all political, ethnic and religious strands of French society.
President Francois Hollande called it France's "best side". But now comes the harder task of channelling that momentum to tackle the security, judicial, social and economic problems being suggested as factors behind a threat still very alive.
"This sense of unity is of course extremely fragile," Catholic daily La Croix warned. "It won't take much for the usual quarrels and acrimony to snuff it out. We can but hope."
Yet cracks in that unity emerged as early as Monday as the French tuned into their regular breakfast media interviews.
While the government has steadfastly made no inferences from the Algerian roots of the Kouachi brothers or African origins of deli killer Amedy Coulibaly, its critics see the killers' ethnicity as relevant to what they see as an overdue immigration debate.
"Immigration is not linked to terrorism but does complicate matters," ex-president Nicolas Sarkozy, who as leader of the conservative opposition marched just behind President Francois Hollande on Sunday, told RTL radio.
"When integration is not working, that creates a problem on our territory," he added.
Sarkozy did not offer solutions. But the theme tallies with his call for immigrants to "wed" France's language and culture - part of his goal of winning back voters from the resurgent far-right National Front in time for the 2017 presidential election.
Such talk feeds into a volatile context where attacks on Muslim targets were reported in the days after the Hebdo killings and where a debate on identity is being fueled by books such as a new novel imagining a Muslim French president in 2022.
Hollande's Socialist government argues equal opportunities are the key to racial harmony but like past governments has yet to tackle the sense of alienation felt for years in the areas around major cities where many immigrants live.
"Those appalling crimes were committed by youths who grew up in our country, which has sometimes not been able to help them build a future for themselves," Economy Minister Emmanuel Macron conceded to reporters at an event on Monday.
French "Patriot act"?
Under pressure from EU partners to cut its public borrowing, Hollande's government is unlikely any time soon to be able to fund the huge "Marshall Plan" regeneration of France's suburbs that past administrations have promised but not delivered.
Equally hard to tackle are perceived weaknesses in France's penal and intelligence structures which Valls on Monday pledged to address urgently.
"We have got to do some work on the prisons, that is a major priority," he told French television of the radicalisation risk in prisons, many of them overcrowded, such as the one where Hebdo killer Cherif Kouachi first met Coulibaly.
With last week's events seen by many as "France's 9/11", calls have grown for a "Patriot Act" similar to that introduced in the United States after the September 2001 attacks to bolster security with wide-ranging surveillance and detention powers.
That is being ruled out for now. But Valls acknowledged shortfalls in surveillance of the killers and said greater resources are needed as well as a review of legal limits on it.
While one of the enduring images from Sunday's march was the sight of Hollande walking arm-in-arm with Germany's Angela Merkel and other EU leaders, France must win concrete backing from its European partners to step up the anti-terrorist fight.
Interior Minister Bernard Cazeneuve called on Sunday for greater cross-border cooperation, saying EU arrangements were not adequate for dealing with the threat. He urged the creation of a joint database on air passenger data currently blocked in the European Parliament on privacy concerns.
In what may appear an irony on the same day as a rally which for many was in support of freedom of expression, Cazeneuve also called on Europe to fight against abusive use of the web to recruit young people for violence and hate speech.
Prosecutors launched an inquiry for apology of terrorism on Monday against the comedian Dieudonne for writing on his Facebook account that he felt "Charlie Coulibaly" - a word play on the "Je suis Charlie" ("I am Charlie") slogan of the vigil.
"We need to work more closely with Internet companies to guarantee the reporting and if possible removal of all content that amounts to an apology of terrorism or calls for violence and hatred," said Cazeneuve before the Dieudonne case.
In coming months, the spotlight may also fall on whether France will maintain its military role in Muslim countries such as Iraq or Mali - interventions cited in a posthumous video by a man identified as Coulibaly as motivating his attack.
In a front page editorial summing up the collective transition which France and its leaders must now make from the trials of the past few days to the future ones, Le Figaro headlined with: "After emotion, courage".